Can a "primary POA" legally banish a secondary "medical POA" from seeing Mom or Dad because of personal disagreements?

I have 2 siblings whom are appointed 1st and 2nd POAs by my parent. Arguments have developed between them... the primary POA is the parent's "favorite", and has always been spoiled by them. The medical POA lived near the parents, and helped them since age 18 or 20 (now age 68); never asking for anything in return. One parent died about 35 years ago.
Both POAs lived near and remained in constant contact with the surviving parent, but the primary POA remained dependent upon the surviving parent, "taking from"... The medical POA always "gave to" the surviving parent in the form of heavy work in maintaining and improving house, shopping, maintaining bank accounts, paying bills, appointments, pet-care, errands. Never receiving payment, nor expecting any payment. There are 6 siblings.
We took care of the parent together for about 2 years... the "primary POA" enforced demands and criticism until 2 siblings bailed out; then 2 years after they left, the Primary POA began harassing and demanding until, one-by-one, the other 3 departed because they couldn't stand the stresses anymore. The parent always took his side, so nothing could be negotiated. About 7 months ago, this Primary POA began to single-handedly care for the parent alone, moving in with the parent. The POA does not inform any siblings of the parent's condition, nor situation; the house has become dirty; the parent receives 1 bath per week and often has feces and urine on the backside, the blinds and curtains are always pulled and the entire question of being welcome depends upon the whim of the primary POA... who manifests signs of paranoia, extreme secrecy and sociopathic behavior. POA has recently taken control of money, bank accounts, acquired a new van etc. Calls to "Adult Services" are now ignored because an agent visited many times before (3-4 years ago) when first 2 siblings complained and 4 were taking care of the parent; yet the situation has deteriorated since POA chased everyone away 7 months ago. POA has refused to allow the Medical POA visiting- rights; and does not notify Med POA when parent is hospitalized. Is this legal? Any advise would be helpful.